This invention relates to a solid electrolytic capacitor and, in particular, relates to a solid electrolytic capacitor with face-down terminals having electrodes directly drawn out to its board mount side from the bottom of a capacitor element, a method of manufacturing such a face-down terminal solid electrolytic capacitor, and a lead frame for use therein.
Solid electrolytic capacitors using tantalum, niobium, or the like as a valve-action metal are small in size, large in capacitance, and excellent in frequency characteristic and have thus been widely used for decoupling in power circuits of CPUs and so on. Following the recent development of portable electronic devices, particularly the advanced functionality thereof, face-down terminal solid electrolytic capacitors of the type where electrodes are directly drawn out to the board mount side have been commercialized.
Japanese Unexamined Patent Application Publication (JP-A) No. 2004-228424 discloses a face-down terminal solid electrolytic capacitor of this type. According to this publication, it is necessary to apply plating to fillet surfaces, recessed from end surfaces of electrode terminals, after cutting of the terminals and therefore there is a problem in terms of reducing the number of processes.
As techniques for solving this problem, the applicant of this application has filed Japanese Patent Application No. 2003-334961 (Japanese Unexamined Patent Application Publication (JP-A) No. 2005-101418) and Japanese Patent Application No. 2004-002180 (Japanese Unexamined Patent Application Publication (JP-A) No. 2005-197457). Although these techniques can solve the foregoing problem of the number of processes relating to the plating, further improvements are needed in other aspects.
For example, there is a problem that electrode terminals come off a casing resin while manufacturing a face-down terminal solid electrolytic capacitor or mounting the product onto a board. This is because it is considered that anchoring forces between the electrode terminals and the casing resin are small and thus the anchor effect therebetween is insufficient. Another problem is that when an anode lead of a capacitor element is welded to a lead frame and then this composite is overmolded with a casing resin, and finally a face-down terminal solid electrolytic capacitor is cut out therefrom, there is no securely welded portion between the anode lead of the capacitor element and the anode terminal in the capacitor so that connection failure may be caused, thereby degrading reliability of the capacitor. This is because it is considered that since the welding margin of the lead frame extends over the anode lead in a longitudinal direction thereof, secure welding cannot be achieved at a precise position.
This situation will be further explained with reference to the drawings. FIGS. 10A to 10C shows a solid electrolytic capacitor with face-down terminals described in JP-A-2005-197457, wherein FIG. 10A is a side view of the capacitor on an anode side thereof, FIG. 10B is a front view of the capacitor where a casing resin on the right side of a line A—A in FIG. 10A is removed, and FIG. 10C is a side view of the capacitor on a cathode side thereof. In FIGS. 10A to 10C, 11 denotes a capacitor element, 12 an anode lead, 73 a face-down anode terminal, 74 a face-down cathode terminal, 76a an anode-side fillet surface having been subjected to plating, 76b a cathode-side fillet surface having been subjected to plating, 79 a generally [-shaped anode terminal cut surface appearing on one side of the capacitor, 17 an insulating resin, 78 a cathode terminal cut surface appearing on the other side of the capacitor, 19 an insulating casing resin, and 20 a conductive adhesive. In FIG. 10B, the fillet surfaces 76a and 76b are formed at positions slightly recessed from the anode terminal cut surface 79 and the cathode terminal cut surface 78, respectively.
Manufacturing processes of this solid electrolytic capacitor with face-down terminals will be described with reference to FIG. 11. FIG. 11 is a flow diagram showing the manufacturing processes of the capacitor shown in FIGS. 10A to 10C. S61 is a process of producing a lead frame, S63 is a process of applying plating to the lead frame, S64 is a process of fixedly joining capacitor elements to the lead frame, S65 is a process of overmolding with a casing resin, and S66 is a process of cutting the casing resin and the lead frame.
FIG. 12 is a front view showing the state where the casing resin on the front side of the plane passing through a center axis of the capacitor and perpendicular to the bottom surface of the lead frame is removed after joining the capacitor element to the lead frame and overmolding them with the casing resin in the foregoing processes shown in FIG. 11. In FIG. 12, 81 denotes an anode terminal forming portion of the lead frame, 82 a cathode terminal forming portion of the lead frame, 23a and 23b cutting planes, respectively, and 84a and 84b recessed portions after cutting to serve as the fillet surfaces, respectively. By providing the plated recessed portions in this manner, the process of plating after cutting becomes unnecessary.
According to the technique of JP-A-2005-101418 or JP-A-2005-197457, the process of plating after cutting is not required so that the productivity is excellent. However, the following problems arise in the process of cutting out the capacitor as a product from the lead frame or thereafter. Specifically, (1) the electrode terminals may come off the casing resin after the cutting due to friction at the time of the cutting and (2) since the anchor effect is insufficient in the casing resin, the electrode terminals may come off when mounting the product onto a board. These problems become significant particularly for the cathode terminal of which the anchor effect with the casing resin is small.
Further, when a flat surface of the anode terminal provided for welding of the anode lead is uniformly planar over the anode lead in a longitudinal direction thereof, it is difficult to accurately control a position of welding with the anode lead. Therefore, the reliability of connection largely changes depending on a position where the anode lead is welded to the flat surface of the anode terminal.